Polyelectrolyte complexes are interpenetrating complexes of one or more predominantly positive polyelectrolytes and one or more predominantly negative polyelectrolytes. The opposite charges on the polymers form ion pairs between chains, holding the chains together. This ion pairing is a type of physical crosslinking.
Polyelectrolyte complexes may be prepared in a straightforward manner by mixing solutions of positive and negative polyelectrolytes. However, the resulting precipitate is gelatinous and difficult to process. The dried complexes, for example, are generally infusible and therefore cannot be injection molded or reformed into articles under elevated temperatures. U.S. Pat. No. 3,546,142 discloses a method for creating solutions of polyelectrolyte complexes using aggressive ternary solvents which are mixtures of salt, water, and organic solvent. Said solutions of dissolved complexes may be cast into films by evaporating the solvent on horizontal plates. Mani et al. (U.S. Pat. No. 4,539,373) point out that the solid polyelectrolyte complexes “are not thermoplastic, i.e., they are not moldable or extrudable, so they must be handled as solutions.” Mani discloses a polyelectrolyte complex comprising nonionic thermoplastic repeat units which can be thermally molded.
U.S. Pat. Nos. 8,114,918; 8,206,822; 8,222,306; 8,283,030; 8,314,158; and 8,372,891, which are incorporated fully by reference, disclose how fully hydrated (i.e., complexes in contact with water) polyelectrolyte complexes may be reformed into shapes without raising the temperature, without the addition of organic solvent, and without the need for dissolution, if they are doped with salt ions to a sufficient extent.
An alternative method for producing ultrathin films (less than about 1 micrometer thick) of polyelectrolyte complex is the multilayering method described by Decher et al. in U.S. Pat. No. 5,208,111, wherein a surface is exposed in an alternating fashion to solutions of positive and negative polyelectrolytes. The resulting films are uniform and conformal, though ultrathin. The process, however, can be unacceptably slow, especially if numerous layers of polyelectrolyte are needed.
The multilayering method may be performed on planar or on curved surfaces. For example, polyelectrolyte multilayers have been deposited on particles as described in U.S. Pat. No. 6,479,146. The resulting thin polyelectrolyte complex completely envelopes the particle. Therefore, as disclosed in U.S. Pat. No. 6,479,146, when the particle is dissolved a hollow capsule with walls comprising polyelectrolyte complex remains.
Capsules are often used to package materials in medicine, pharmacy, sensors, microreaction chambers, and catalysts as described in De Geest et al Chem. Soc. Rev. 36, 636-649 (2007) and Becker et al. Small, 6, 1836-1852 (2010).
The method of preparing capsules comprising polyelectrolyte complex disclosed in U.S. Pat. No. 6,479,146 requires building up said complex layer by layer with consecutive depositions of positive and negative polyelectrolytes. After the polyelectrolyte complex is formed, in whole or in part, the particle at the core must now be dissolved to yield the hollow capsule. Often, the material in the core cannot be transported out of the capsule unless the walls are thin. Loading the capsule post synthesis requires yet another processing step.
The process of multilayering, core dissolution and core loading is a time consuming and repetitive process. A much faster method of preparing hollow polyelectrolyte capsules is needed.